1,3-Propanediol is a monomer having potential utility in the production of polyester fibers and the manufacture of polyurethanes and cyclic compounds.
A variety of chemical routes to 1,3-propanediol are known. For example ethylene oxide may be converted to 1,3-propanediol over a catalyst in the presence of phosphine, water, carbon monoxide, hydrogen and an acid, by the catalytic solution phase hydration of acrolein followed by reduction, or from compounds such as glycerol, reacted in the presence of carbon monoxide and hydrogen over catalysts having atoms from group VIII of the periodic table. Although it is possible to generate 1,3-propanediol by these methods, they are expensive and generate waste streams containing environmental pollutants.
It has been known for over a century that 1,3-propanediol can be produced from the fermentation of glycerol. Bacterial strains able to produce 1,3-propanediol have been found, for example, in the groups Citrobacter, Clostridium, Enterobacter, Ilyobacter, Klebsiella, Lactobacillus, and Pelobacter. In each case studied, glycerol is converted to 1,3-propanediol in a two step, enzyme catalyzed reaction sequence. In the first step, a dehydratase catalyzes the conversion of glycerol to 3-hydroxypropionaldehyde (3-HPA) and water, Equation 1. In the second step, 3-HPA is reduced to 1,3-propanediol by a NAD+-linked oxidoreductase, Equation 2. The 1,3-propanediol is not metabolized further and, as a result,Glycerol→3-HPA+H2O  (Equation 1)3-HPA+NADH+H+→1,3-Propanediol+NAD+  (Equation 2)accumulates in the media. The overall reaction consumes a reducing equivalent in the form of a cofactor, reduced β-nicotinamide adenine dinucleotide (NADH), which is oxidized to nicotinamide adenine dinucleotide (NAD+).
In Klebsiella pneumonia, Citrobacter freundii, and Clostridium pasteurianum, the genes encoding the three structural subunits of glycerol dehydratase (dhaB1-3 or dhaB, C and E) are located adjacent to a gene encoding a specific 1,3-propanediol oxidoreductase (dhaT) (see FIG. 1). Although the genetic organization differs somewhat among these microorganisms, these genes are clustered in a group which also comprises orfX and orfZ (genes encoding a dehydratase reactivation factor for glycerol dehydratase), as well as orfY and orfW (genes of unknown function). The specific 1,3-propanediol oxidoreductases (dhaT's) of these microorganisms are known to belong to the family of type III alcohol dehydrogenases; each exhibits a conserved iron-binding motif and has a preference for the NAD+/NADH linked interconversion of 1,3-propandiol and 3-HPA. However, the NAD+/NADH linked interconversion of 1,3-propandiol and 3-HPA is also catalyzed by alcohol dehydrogenases which are not specifically linked to dehydratase enzymes (for example, horse liver and baker's yeast alcohol dehydrogenases (E.C. 1.1.1.1)), albeit with less efficient kinetic parameters. Glycerol dehydratase (E.C. 4.2.1.30) and diol[1,2-propanediol] dehydratase (E.C. 4.2.1.28) are related but distinct enzymes that are encoded by distinct genes. Diol dehydratase genes from Klebsiella oxytoca and Salmonella typhimurium are similar to glycerol dehydratase genes and are clustered in a group which comprises genes analogous to orfX and orfZ (Daniel et al., FEMS Microbiol. Rev. 22, 553 (1999); Toraya and Mori, J. Biol. Chem. 274, 3372 (1999); GenBank AF026270).
The production of 1,3-propanediol from glycerol is generally performed under anaerobic conditions using glycerol as the sole carbon source and in the absence of other exogenous reducing equivalent acceptors. Under these conditions, in e.g., strains of Citrobacter, Clostridium, and Klebsiella, a parallel pathway for glycerol operates which first involves oxidation of glycerol to dihydroxyacetone (DHA) by a NAD+- (or NADP+-) linked glycerol dehydrogenase, Equation 3. The DHA, following phosphorylation to dihydroxyacetone phosphate (DHAP) by a DHA kinase (Equation 4),Glycerol+NAD+→DHA+NADH+H+  (Equation 3)DHA+ATP→DHAP+ADP  (Equation 4)becomes available for biosynthesis and for supporting ATP generation via e.g., glycolysis. In contrast to the 1,3-propanediol pathway, this pathway may provide carbon and energy to the cell and produces rather than consumes NADH.
In Klebsiella pneumoniae and Citrobacter freundii, the genes encoding the functionally linked activities of glycerol dehydratase (dhaB), 1,3-propanediol oxidoreductase (dhaT), glycerol dehydrogenase (dhaD), and dihydroxyacetone kinase (dhaK) are encompassed by the dha regulon. The dha regulon, in Klebsiella pneumoniae and Citrobacter freundii, also encompasses a gene encoding a transcriptional activator protein (dhaR). The dha regulons from Citrobacter and Klebsiella have been expressed in Escherichia coli and have been shown to convert glycerol to 1,3-propanediol.
Neither the chemical nor biological methods described above for the production of 1,3-propanediol are well suited for industrial scale production since the chemical processes are energy intensive and the biological processes are limited to relatively low titer from the expensive starting material, glycerol. These drawbacks could be overcome with a method requiring low energy input and an inexpensive starting material such as carbohydrates or sugars, or by increasing the metabolic efficiency of a glycerol process. Development of either method will require the ability to manipulate the genetic machinery responsible for the conversion of sugars to glycerol and glycerol to 1,3-propanediol.
Biological processes for the preparation of glycerol are known. The overwhelming majority of glycerol producers are yeasts but some bacteria, other fungi and algae are also known. Both bacteria and yeasts produce glycerol by converting glucose or other carbohydrates through the fructose-1,6-bisphosphate pathway in glycolysis or the Embden Meyerhof Parnas pathway, whereas, certain algae convert dissolved carbon dioxide or bicarbonate in the chloroplasts into the 3-carbon intermediates of the Calvin cycle. In a series of steps, the 3-carbon intermediate, phosphoglyceric acid, is converted to glyceraldehyde 3-phosphate which can be readily interconverted to its keto isomer dihydroxyacetone phosphate and ultimately to glycerol.
Specifically, the bacteria Bacillus licheniformis and Lactobacillus lycopersica synthesize glycerol, and glycerol production is found in the halotolerant algae Dunaliella sp. and Asteromonas gracilis for protection against high external salt concentrations. Similarly, various osmotolerant yeasts synthesize glycerol as a protective measure. Most strains of Saccharomyces produce some glycerol during alcoholic fermentation, and this can be increased physiologically by the application of osmotic stress. Earlier this century commercial glycerol production was achieved by the use of Saccharomyces cultures to which “steering reagents” were added such as sulfites or alkalis. Through the formation of an inactive complex, the steering agents block or inhibit the conversion of acetaldehyde to ethanol; thus, excess reducing equivalents (NADH) are available to or “steered” towards DHAP for reduction to produce glycerol. This method is limited by the partial inhibition of yeast growth that is due to the sulfites. This limitation can be partially overcome by the use of alkalis that create excess NADH equivalents by a different mechanism. In this practice, the alkalis initiated a Cannizarro disproportionation to yield ethanol and acetic acid from two equivalents of acetaldehyde.
The gene encoding glycerol-3-phosphate dehydrogenase (DAR1, GPD1) has been cloned and sequenced from S. diastaticus (Wang et al., J. Bact. 176, 7091-7095 (1994)). The DAR1 gene was cloned into a shuttle vector and used to transform E. coli where expression produced active enzyme. Wang et al. (supra) recognize that DAR1 is regulated by the cellular osmotic environment but do not suggest how the gene might be used to enhance 1,3-propanediol production in a recombinant microorganism.
Other glycerol-3-phosphate dehydrogenase enzymes have been isolated: for example, sn-glycerol-3-phosphate dehydrogenase has been cloned and sequenced from Saccharomyces cerevisiae (Larason et al., Mol. Microbiol. 10, 1101 (1993)) and Albertyn et al. (Mol. Cell. Biol. 14, 4135 (1994)) teach the cloning of GPD1 encoding a glycerol-3-phosphate dehydrogenase from Saccharomyces cerevisiae. Like Wang et al. (supra), both Albertyn et al. and Larason et al. recognize the osmo-sensitivity of the regulation of this gene but do not suggest how the gene might be used in the production of 1,3-propanediol in a recombinant microorganism.
As with G3PDH, glycerol-3-phosphatase has been isolated from Saccharomyces cerevisiae and the protein identified as being encoded by the GPP1 and GPP2 genes (Norbeck et al., J. Biol. Chem. 271, 13875 (1996)). Like the genes encoding G3PDH, it appears that GPP2 is osmosensitive.
Although a single microorganism conversion of fermentable carbon source other than glycerol or dihydroxyacetone to 1,3-propanediol is desirable, it has been documented that there are significant difficulties to overcome in such an endeavor. For example, Gottschalk et al. (EP 373 230) teach that the growth of most strains useful for the production of 1,3-propanediol, including Citrobacter freundii, Clostridium autobutylicum, Clostridium butylicum, and Klebsiella pneumoniae, is disturbed by the presence of a hydrogen donor such as fructose or glucose. Strains of Lactobacillus brevis and Lactobacillus buchner, which produce 1,3-propanediol in co-fermentations of glycerol and fructose or glucose, do not grow when glycerol is provided as the sole carbon source, and, although it has been shown that resting cells can metabolize glucose or fructose, they do not produce 1,3-propanediol (Veiga D A Cunha et al., J. Bacteriol., 174, 1013 (1992)). Similarly, it has been shown that a strain of Ilyobacter polytropus, which produces 1,3-propanediol when glycerol and acetate are provided, will not produce 1,3-propanediol from carbon substrates other than glycerol, including fructose and glucose (Steib et al., Arch. Microbiol. 140, 139 (1984)). Finally, Tong et al. (Appl. Biochem. Biotech. 34, 149 (1992)) taught that recombinant Escherichia coli transformed with the dha regulon encoding glycerol dehydratase does not produce 1,3-propanediol from either glucose or xylose in the absence of exogenous glycerol.
Attempts to improve the yield of 1,3-propanediol from glycerol have been reported where co-substrates capable of providing reducing equivalents, typically fermentable sugars, are included in the process. Improvements in yield have been claimed for resting cells of Citrobacter freundii and Klebsiella pneumoniae DSM 4270 co-fermenting glycerol and glucose (Gottschalk et al., supra.; and Tran-Dinh et al., DE 3734 764); but not for growing cells of Klebsiella pneumoniae ATCC 25955 co-fermenting glycerol and glucose, which produced no 1,3-propanediol (I-T. Tong, Ph.D. Thesis, University of Wisconsin-Madison (1992)). Increased yields have been reported for the cofermentation of glycerol and glucose or fructose by a recombinant Escherichia coli; however, no 1,3-propanediol is produced in the absence of glycerol (Tong et al., supra.). In these systems, single microorganisms use the carbohydrate as a source of generating NADH while providing energy and carbon for cell maintenance or growth. These disclosures suggest that sugars do not enter the carbon stream that produces 1,3-propanediol.
Recently, however, the conversion of carbon substrates, other than glycerol or dihydroxyacetone, to 1,3-propanediol by a single microorganism that expresses a dehydratase enzyme has been described (U.S. Pat. No. 5,686,276; WO 9821339; WO 9928480; and WO 9821341 (U.S. Pat. No. 6,013,494)). A specific deficiency in the biological processes leading to the production of 1,3-propanediol from either glycerol or glucose has been the low titer of the product achieved via fermentation; thus, an energy-intensive separation process to obtain 1,3-propanediol from the aqueous fermentation broth is required. Fed batch or batch fermentations of glycerol to 1,3-propanediol have led to final titers of 65 g/L by Clostridium butyricum (Saint-Amans et al., Biotechnology Letters 16, 831 (1994)), 71 g/L by Clostridium butyricum mutants (Abbad-Andaloussi et al., Appl. Environ. Microbiol. 61, 4413 (1995)), 61 g/L by Klebsiella pneumoniae (Homann et al., Appl. Bicrobiol. Biotechnol. 33, 121 (1990)), and 35 g/L by Citrobacter freundii (Homann et al., supra). Fermentations of glucose to 1,3-propanediol that exceed the titer obtained from glycerol fermentations have not yet been disclosed.
The problem that remains to be solved is how to biologically produce 1,3-propanediol, with high titer and by a single microorganism, from an inexpensive carbon substrate such as glucose or other sugars. The biological production of 1,3-propanediol requires glycerol as a substrate for a two-step sequential reaction in which a dehydratase enzyme (typically a coenzyme B12-dependent dehydratase) converts glycerol to an intermediate, 3-hydroxypropionaldehyde, which is then reduced to 1,3-propanediol by a NADH- (or NADPH) dependent oxidoreductase. The complexity of the cofactor requirements necessitates the use of a whole cell catalyst for an industrial process that utilizes this reaction sequence for the production of 1,3-propanediol.